UV photodetectors and light emitters find numerous uses including applications in the defense, commercial, and scientific arenas. These include, for example, covert space-to-space communications, missile threat detection, chemical and biological threat detection and spectroscopy, UV environmental monitoring, and germicidal cleansing. UV photodetectors and light emitters operating in the solar blind region are of special interest. The solar blind region corresponds to the spectral UV region where strong upper atmospheric absorption of solar radiation occurs, generally at wavelengths from about 200 nm to about 290 nm. This creates a natural low background window for detection of man-made UV sources on and proximate to the earth's surface.
Semiconductor materials having a 25° C. band gap of about 4 eV to 6 eV have been used to sense or generate solar blind UV radiation. Conventional approaches have used compound semiconductor materials such as AlGaN, MgZnO, or BeZnO, which generally have wurtzite (hexagonal) lattice structures. AlGaN is known to suffer from various problems including lattice cracking due to strain, generally high dislocation density, and lattice mismatch with respect to the layer it is grown on (all such effects are generally interrelated). High dislocation density undesirably reduces internal quantum efficiency.
The crystal structure of MgxZn1-xO can be cubic or wurtzite, depending on the Mg/Zn ratios. High % Mg compositions result in a cubic structure while low % Mg compositions result in a Wurtzite structure. However, the crystal structure difference and large lattice mismatch between ZnO (wurtzite, 3.25 Å) and MgO (rock salt, 4.22 Å) causes phase segregation in MgxZn1-xO with Mg compositions between about 37%<x<62% BeZnO is generally considered a somewhat more promising semiconductor material, but has experienced doping difficulties, particularly difficulties in obtaining high mobility and stable p-type doping.
Epitaxial monocrystalline ZnO on sapphire (Al2O3) substrates is known to be a relative low cost PD option. Process temperatures used to obtain crystalline ZnO commonly exceed 500° C. There is a large in-plane lattice mismatch (18%) between c-oriented ZnO and sapphire, typically resulting in a high dislocation density of generally more than 109 cm−2 for epitaxial ZnO layers, which may lead to low responsivity for photodiodes made from or on such layers.